1. Field of the Invention
The present invention relates generally to radio signal antennas and more particularly relates to phased array antennas with digital antenna pattern control.
2. Description of the Prior Art
It is well known in the prior art that antennas for radiating and receiving radio signals may be formed from several individual antenna elements. By arranging the antenna elements with specific geometry, and combining signals associated with the individual elements with a specific phase and amplitude relationship, the individual elements cooperate to form a unitary antenna structure.
Each of the individual antenna elements in such an antenna (in a transmit application) radiates a common signal which is common in frequency, but altered in amplitude and phase from the other elements. As a result, the individual signals combine in space at varying phase and amplitude levels to create an antenna pattern. The signal combination essentially follows a three dimensional vector addition function. The combination of signals which are in phase results in signal lobes. The cancellation of signals which are out of phase (180.degree.) results in signal nulls. For all phase angles in between these extremes, partial cancellation occurs which shapes the signal lobes. The resultant signal is referred to as the antenna pattern. The antenna pattern is characterized by the number of lobes, the magnitude of the lobes (gain), the direction of the lobes and the relative magnitude of the lobes in differing directions (directivity).
In multi-element array antennas, the gain, directivity and lobe direction may be varied by controlling the phase of the signals driving the individual elements. This type of antenna is conventionally referred to as a phased array. An in depth treatment of conventional phased arrays is presented in The Radar Handbook, Second Edition, Edited by Merrill Skolnik, published in 1990 by McGraw-Hill, which is incorporated herein by reference.
Phased arrays may be formed as linear arrays (FIG. 1), planar arrays (FIG. 2), or conforming arrays (FIG. 3). The linear array shown in FIG. 1 is capable of producing an antenna pattern which can be rotated along (scanned) a two dimensional plane by varying the phase of the signals driving each of the antenna elements 2. The planar and conforming arrays are capable of scanning in three dimensional space by appropriately driving the individual antenna elements 2.
Regardless of the chosen array geometry, it is required that the signal along each path between a signal source and the antenna elements have a controlled phase and magnitude in order to form a desired antenna pattern. This is achieved by controlling signal power division ratios and the phase shift in the electrical transmission path between the signal source and each antenna element. A structure which performs this function is generally referred to as an antenna feed.
FIG. 4 illustrates a conventional "corporate feed" antenna feed topology. In a corporate feed, a signal source 4 simultaneously drives, in parallel, each of the antenna elements 2. In a corporate feed, the length of each transmission line segment 6 is the same for each antenna element 2. The phase of the signal driving each element is controlled by an analog phase shift network 8. For a variable antenna pattern, each antenna element 2 will have an individually controllable analog phase shift network 8.
An alternative antenna feed network, a series feed, is illustrated in FIG. 5. In the conventional series feed network, a series of antenna elements 2 are connected in a single transmission line 6 with a built in phase progression between the antenna elements 2. The phase progression is determined in part by the length of the transmission line 6 (physical path length) between successive antenna elements 2. The phase of the signal at each element 2 is related to the electrical path length between antenna elements 2. The electrical path length, expressed in wavelengths, changes with frequency for a fixed physical path length. Therefore, the phase progression between antenna elements 2 in a series feed varies with frequency. For variable antenna patterns, variable analog phase shift networks 8 may be inserted between the antenna elements 2.
A third conventional antenna feed network, a space feed network, is illustrated in FIG. 6. In the space feed network, a source antenna 10 is electrically connected to a signal source 4. The source antenna 10 radiates a signal received from the signal source 4. The radiated signal is received by a series of pickup elements 12. The received signals are then coupled through phase and amplitude shift networks 9 to the antenna elements 2 for transmission.
The antenna feed topologies illustrated in FIGS. 4, 5 and 6 each require the use of analog phase shift networks in line with each antenna element to achieve dynamic antenna pattern control or scanning. Analog phase shift networks require tuning during manufacturing and are not directly controllable by a digital signal from a computer. Further, analog circuitry is subject to significant parametric variation with changing environmental conditions, such as ambient temperature. In high power signal transmission applications requiring high speed variation of the antenna pattern, the phase shift network must be implemented at a low signal power level. The phase shifted signals must then be amplified subsequently for each antenna element. Phase shifting before final power amplification avoids significant power loss caused by the high speed analog phase shifters. The combination of these factors makes analog phase shifters difficult to manufacture and complex to control in an automated beam scanning system.
The problems associated with analog phase shift networks have been addressed in receiving antenna systems by the implementation of digitally beam formed (DBF) receiver antennas. A typical receive-only DBF antenna is illustrated in FIG. 7. In a receive DBF antenna, the antenna elements 2 of the phase array are coupled to analog to digital (AID) converters 20. Typically, signal amplifiers will be interposed between the antenna elements 2 and A/D converter 20, to increase the received signal level. Signal mixers may also be interposed between the antenna elements 2 and A/D converter 20 to convert the frequency of the received signals into the operating range of the A/D converter 20.
Each A/D converter 20 digitizes the received signals from antenna elements 2 and presents a digital signal to a digital processor 22. The digital processor 22 mathematically alters the magnitude and phase of the received digital signals. The digital processor 22 then combines these altered signals to synthesize the desired antenna pattern. In this fashion, a receive antenna is formed without the need for analog phase converter. However, the DBF antenna of FIG. 7 is only applicable for radio signal receiving systems, not transmission systems.